Idk why it doesn't mention anywhere that OnMouse***() functions DO NOT work on UI elements, but they don't. You have to either use the EventTrigger component (PointerEnter event) or you can do what codeelemental said and implement the corresponding interfaces.

Also, you are probably looking for OnMouseEnter, as OnMouseOver is called every frame.

Another thought is, it seems you are trying to make a hint text. You might want to try and change the button transition type to animation instead of color, and enable the text object in the "highlighted" animation clip (you must also make the text a child of the button). You could even do a nice smooth expansion of a chat box or something.